Chicago Teachers Union members started voting Wednesday on whether to authorize a strike.

Teachers have until Friday to cast their ballots

This is the second CTU strike authorization vote. In December, 88 percent of members voted to walk out.

But the CTU wants another vote, to make their case airtight. If at least 75 percent of members vote in the affirmative, the strike could happen as early as mid-October.

The union is angry over pay and pension. Mayor Rahm Emanuel has said teachers will get a 13 percent pay raise over the life of this contract. But the district would eliminate a 7 percent pension payment.

"It's their pension they are going to pay into, that they themselves and their families will get," the mayor said.

CTU President Karen Lewis has said she is going to defend teachers' rights to be paid.

Chicago Public Schools told ABC7 Eyewitness News they think a strike can be averted, and they are going to continue to make sure CPS students' education is not interrupted.